Validate default_kernel global property on set

fixes: QubesOS/qubes-issues#8992

qubes/app.py

@@ -1593,3 +1593,12 @@ def on_property_set_default_dispvm(self, event, name, newvalue,
                 # resetting dispvm to its default value
                 vm.fire_event('property-reset:default_dispvm',
                               name='default_dispvm', oldvalue=oldvalue)
+
+    @qubes.events.handler('property-pre-set:default_kernel')
+    def on_property_pre_set_default_kernel(self, event, name, newvalue,
+                                           oldvalue=None):
+        # pylint: disable=unused-argument
+        kernels = [kernel.vid for kernel in self.pools['linux-kernel'].volumes]
+        if not newvalue in kernels:
+            raise qubes.exc.QubesException(
+                    'Invalid kernel: {} !'.format(newvalue))